Summary: Changing the IM L, P and Y damping loops to roll off sooner reduces noise in DARM between 20-30 Hz, reduces noise in REFL WFS, removes IM damping coherence with REFL WFS.

Today I tried changing our IM damping loops over to the ones Livingston uses for their IMs. This was mostly successful, however, the lack of control in the yaw damping loops caused lots of ringing in the INP1 yaw loop when we went to ENGAGE_ASC_FOR_FULL_IFO. In that moment, I increased the gain on the IM yaw damping loops (all four), by a factor of 10. This helped, but was another clue that the yaw filter L1 uses would not be successful for us. My proposal, which we agreed on in the commissioning meeting, was that we should use the zpk 0:15,15 and ellip7 filter that is used for the IM L damping loop for all three loops. This is not the most ideally designed situation, but would help considerably. I kept the same gains we were using for the old damping loops, L: -20, P: -0.02, Y: -0.04. While at NLN lock, I switched over to these new filters, and measured coherences and the noise in the REFL WFS. Sheila and I saw that it appeared that DARM improved between 20 and 30 Hz. (first attachment) Yellow trace shows DARM with the new IM damping loops, red shows DARM with the old IM damping loops. The grey trace shows DARM with the old loops before the HAM1 vent. I measured this in a chop test several times and saw consistent improvement in DARM from 20-30 Hz with the new damping loops. This plot (second attachment) also shows the change in the REFL WFS, red showing the old WFS spectrum with the old damping loops, and blue showing the new WFS spectrum with the new damping loops. I did have a plot showing that there is no more coherence in the WFS and IM damping, but did not save the data before a workstation restart. I will post the coherence plots tomorrow!

Tomorrow during maintenance I will also work on getting an OLG of the IMs, especially to check the phasing.